# §150.0(c): Wall insulation
What it requires
Every above-grade exterior wall on a California residence must carry insulation in the cavity (the space between the studs):
- 2x4 framed walls — R-13 cavity mandatory minimum.
- 2x6 framed walls — R-19 cavity mandatory minimum.
- Below-grade walls (basement / partial-grade) — R-13 mandatory.
- Mass walls (concrete, masonry, ICF) — R-8 continuous insulation minimum, location of the CI varies (exterior preferred in hot climates, interior in cold).
§150.0(c) is the floor; the prescriptive package in §150.1(c)1 stacks additional requirements on top (continuous insulation sheathing, higher cavity R in 2x6 walls). A performance trade-off can drop a project from the prescriptive R-21+R5 down toward the mandatory floor — but never below it.
When it applies
- Newly constructed residential, on every exterior wall.
- Additions, on the new addition's walls.
- Alterations whenever the wall assembly is opened up (re-siding without opening the cavity does not trigger it).
How we use it
The Confirm step applies the per-climate-zone prescriptive wall construction automatically. All three design options (Prescriptive, High-Performance Envelope, Hybrid) clear the §150.0(c) mandatory floor by construction — the floor is rarely binding in California where the prescriptive numbers are well above it.
Where it matters: alteration scope. If you're opening up an existing wall to re-route plumbing or add an electrical run, the AHJ inspector expects the cavity to be re-insulated to at least §150.0(c) before the wall is closed up.
Common gotchas
- Garage-to-house common walls — they're considered exterior walls because the garage is unconditioned. R-13 cavity is the floor and most builders default to R-19 to make the §150.1(c)1 prescriptive number on the same studs.
- Knee walls between conditioned and unconditioned attic space — frequently missed at insulation rough-in because the rafter framing covers them.
- ICF walls priced as "R-25 effective" — the effective number counts the concrete mass; the mandatory R-8 floor is on the continuous foam alone.